I don't think it's so much Apple trying to sell people the 15" as it is actual issues with heat and power consumption. The first revision of the rMBP 13" had dual heatsinks, they've now gotten that down to a single heatsink-fan assembly, marking an improvement in power consumption in the 13, which means that if they were to release a QC version, it would need all that cooling put back, and along with battery life being dramatically hindered, taking a few points off of the machines spec list.
On top of all that a QC i7 is no good without a discrete GPU for most people, therefore that would have to be thrown in also, which is nigh on impossible at this point with a 13" notebook as thin as an rMBP.
My reasons for this is that there is a niche of people who want that kind of a notebook and are willing to pay for it, to have it in 13".